Digital signal transmission system

Abstract

A transmission system of the kind comprising, on emission, a code converter for coding said signals into variable length words according to their occurrence probabilities, the word rate being determined by an emission clock, a synchronizing word generator operating at a frequency bound to said emission frequency, means for multiplexing words from the code converter and synchronizing-words, and an elastic memory connected to said multiplexing means, and, on reception, an elastic memory connected to said emission elastic memory, reading on emission and writing on reception being carried out at a rate determined by a transmission clock, means for detecting synchronizing-words at the output of said reception-memory and a decoder for deriving said signals from said variable length words, said system further comprising, on emission, means for supplying to said multiplexing means the drift of said emission frequency with respect to a frequency bound to said transmission frequency, and, on reception, a controlled oscillator supplying a timing signal for reading said reception memory, means for detecting said drift, said means being controlled by the synchronization-detection; and means for recovering said emission frequency from said drift and said frequency bound to the transmission frequency, said recover means controlling said oscillator.

What we is claim is: 
     
       1. A transmission system of the kind for transmitting and receiving digital signals over a transmission line, said system comprising: for transmission, a code converter for coding said signals into variable length words according to their occurrence probabilities, the word rate being determined by an emission clock for establishing an emission frequency, a synchronising word generator operating at a frequency bound to said emission frequency, means for multiplexing words from the code converter and the synchronising word generator forming variable length words, and a first elastic memory connected between said multiplexing means and said transmission line, and for reception, a second elastic memory connected to said transmission line, reading on transmission and writing on reception being carried out at a rate determined by a transmission clock, means for detecting synchronising words at the output of said second elastic memory and a decoder for deriving digital signals from said variable length words from said second elastic memory, said system for transmission further comprising, for transmission, means for supplying to said multiplexing means a signal representative of the drift of said emission frequency with respect to a frequency bound to said transmission frequency, and, for reception, a controlled oscillator supplying a timing signal for reading said reception memory, means for detecting said signal representative of the drift, said means for detecting said signal being controlled by said means for detecting synchronising words; and means for recovering said emission frequency from said drift and said frequency bound to the transmission frequency, said recover means controlling said oscillator.   
     
     
       2. A digital transmission system according to claim 1, wherein the digital signals are supplied to said code converter by a differential PCM coder, comprising switch means having first and second inputs connected to said code converter and said differential PCM coder, respectively, and outputs connected to said multiplexing means, and a third input, connected to said first elastic memory, for receiving a signal representative of the load level thereof, said switch means delivering signals from said coder when said load level is below a predetermined low level or when the average length of words from said code converter is above that of words from said coder, and switching off transmission when said load level is above a predetermined high level, said switch means delivering at one of its outputs a signal representative of the selection carried out, said signal being detected on reception by said drift detecting means and fed to said decoder.